Imparting flame retardancy to polyolefin compositions which have been extensively used as an insulating material for electric wires, cables and electric appliances, has conventionally been achieved by adding a halogen compound and antimony trioxide to a polyolefin. However, the resulting compositions contain a halogen, and therefore generate, during burning, a halide gas which is harmful to the human body and corodes metals. Moreover, these compositions produce, during burning, a large amount of fumes, making visibility poor. This has imposed a remarkable restriction with respect to evacuation of people and fire fighting activity during fire.
Hence, it has recently become a strong desire for safety that these resin compositions produce no halogen-based gas upon burning. Under such a situation, hydrated metal compounds have come to draw attention as an inorganic flame retardant, because their fuming property and harmfulness are very low.
Resin compositions containing a large amount of magnesium hydroxide, aluminum hydroxide or the like for improving flame retardancy have been proposed in recent years. However, these resin compositions have the following drawbacks.
(1) Magnesium hydroxide reacts with moisture and carbon dioxide present in the air to produce magnesium carbonate. Accordingly, on the surface of a resin composition containing magnesium hydroxide, there appear white crystals of magnesium carbonate.
(2) Magnesium hydroxide and aluminum hydroxide have very low resistances to acids and alkalis. Therefore, when a resin composition containing magnesium hydroxide or aluminum hydroxide is in contact with an acid or an alkali, magnesium hydroxide or aluminum hydroxide readily dissolves in the acid or alkali at the surface of the resin composition. It dissolves in a large amount even in a weak acid having a pH of about 2 and, as a result, is liable to cause deterioration of the resin composition surface.
These drawbacks have delayed the practical applications of the resin compositions containing magnesium hydroxide, aluminum hydroxide or the like to electric wires and cables.
In connection with the present invention, Japanese Patent Application Nos. 129064/1986 and 138316/1986 state as follows.
A silane-grafted polymer causes crosslinking without requiring any catalyst. More specifically, hydroxyl groups (--OH) on the surface of a hydrated metal compound and silanol groups (.fwdarw.Si--OH) of a silane-grafted polymer cause a hydrolytic condensation reaction to form strong silanol linkages [--Si--O--M.sub.m O.sub.n (M is a metal)], under the influences of (a) a small amount of water present on the surface of the hydrated metal compound and (b) the hydrated metal compound which acts similarly to an organic metal compound catalyst and in the presence of a frictional heat generated during kneading; in this reaction, no catalyst is required. The above two patent applications further state that said strong silanol linkages are effective in providing a resin composition with excellent shape retention at high temperatures, improved drip prevention during burning and improved mechanical properties (e.g., improved whitening on bending, and improved wear resistance). The patent applications also state in the Examples that the strong silanol linkages bring about improvements in mechanical properties, surface properties, thermal properties including heat deformation, and burning properties including oxygen index.
As olefinic resins, there have conventionally been available polyethylene, polypropylene, ethylene-alpha-olefin copolymers, ethylene-vinyl acetate copolymer, ethylene-ethyl acrylate copolymer, and polyolefins modified with an unsaturated dicarboxylic acid which are obtained, for example, by reacting a liquid hydrocarbon (e.g. a liquid polybutadiene) as a polymer skeleton with maleic anhydride in the presence of a radical-generating agent such as a peroxide. A number of olefinic resins are being marketed in Japan also. The addition of such a modified olefinic resin to other resins or metal powders for imparting an adhesive property to the latter has been tried and reported in many papers. On this point, a number of patent applications have been filed. The examples are Japanese Patent Publication Nos. 5788/1987 and 9135/1987.
As another example of maleic anhydride utilization, maleic anhydride has been added to thylene-alpha-olefin copolymers in the presence of a peroxide or the like to improve the latter's heat resistance and mechanical strength, as described in Japanese Patent Application Kokai (Laid-Open) Nos. 10151/1987 and 11745/1987. In this approach, the polar group of maleic anhydride is added to a non-polar olefinic resin to merely improve the latter's affinity with a filler and to finally improve the mechanical strength and heat resistance of the latter. Therefore, the purpose of this approach is different from that of an approach intended by the present invention wherein siloxane linkages are formed between a silane-grafted polymer and water of a hydrated metal compound and further complex salt linkages are formed between the hydrated metal compound and a dicarboxylic acid or dicarboxylic acid anhydride derivative.